Aim: To bring together leading researchers using different but potentially complementary techniques for determining the structure of macromolecular assemblies. To produce a definitive volume on this subject first as a Study Book of galleys of posters and papers presented, then [within 2 months] as a final hardback proceedings book of corrected texts and edited transcribed discussion. Method: The Biophysical Discussions use a unique format whereby (a) the complete texts of all contributions are circulated in advance to all participants; (b) the actual meeting time is devoted entirely to discussion (not formal talks); (c) a transcription of plenary debate is made and incorporated into the final proceedings volume. Authors of the 18 plenary papers and 60 posters are selected competitively from abstracts submitted in response to a general international call. Participants are limited to 200. Texts of all papers, including 2-page articles corresponding to the poster presentations and the edited transcripts of the plenary debate, comprise a final proceedings book, fully indexed, also published as the January, 1986 issue of the Biophysical Journal, ensuring ease of reference and wide readership. This conference is fifth in a series of highly successful discussions. Long term objectives: Our primary hope for this meeting is that it will teach people who use particular structural probes to consider results from other techniques, a collaboration that is essential to building a complete picture of a macromolecular assembly. In choosing to focus on molecular assemblies rather than individual molecules, we recognize a level of complexity that is difficult yet now increasingly tractable. We also recognize that these assemblies are functional units whose structural aspects might be critically instructive to those whose primary concern is function. Circulated advance galleys of all discussion texts will enhance exchange among practioners of various techniques, as each group will have a chance to study the vocabulary as well as the data of others before the actual conference. This leads to useful (transcribed) debate.